1. Field of Invention
The present invention relates to improved electrophotographic apparatus and method for controlling electrical memory effects in reusable photoconductive members. More specifically the invention relates to apparatus and techniques for substantially reducing a form of electrical fatigue, occurring in such films, which causes a "residual image" of a previously copied document in subsequent copies of another document.
2. Description of Prior Art
The residual image phenomenon, noted above, is observed as a faint image of a previous document in initial copies of a new document after the previous document has been repeatedly imaged on the reusable photoconductive insulator member, i.e., after that member has been cyclically charged overall and discharged, repeatedly in registry, by the light pattern from the previous document. This residual image effect is believed to be caused by the accumulation of electrons trapped within the volume of the photoconductor in imagewise pattern corresponding to the dark portion of the previous document image. The speed (rate of discharge per unit exposure) of the photoconductor is decreased by this accumulation of trapped electrons so that, upon exposure to a new document, the area of the photoconductive member associated with the previous document pattern is discharged less than other photoconductor portions and is developed with toner as a background image. It will be readily appreciated that such a background image is detractive from the esthetic viewpoint; however, the provision of previous document information in the subsequent document copies presents an even more serious problem when proprietary information is embodied in the previous document.
It is well known that fatigue of the type causing the residual image effect in photoconductive insulator members can be relieved to some extent by application of infrared radiation to, or otherwise heating, such members or by an overall flooding of such members with light (see for example U.S. Pat. No. 2,863,767 and Electrophotography by R. M. Schaffert, 2nd Edition, 1966, page 87). Also, it has been noted that some regeneration of such a fatigued member can be effected by application of an electrostatic charge, of polarity opposite that of the primary (sensitizing) charge, at some time after the development step and before any subsequent sensitizing step of a copy cycle (see for example U.S. Pat. No. 2,741,959). However, in certain electrophotographic apparatus, e.g., a high speed copier/duplicator, in which a photoconductive insulator member is rapidly exposed a large number of times to the same image, the residual image problem is more pronounced; and the above-noted prior art techniques have been found impractical and/or to inadequately eliminate residual image, at least in certain such members.